The invention relates to an arrangement for and a method of recording and/or reproducing data multiplexed with a television picture signal.
As is well known, data signals are broadcast during selected lines in the vertical blanking interval of television signals, this service being known as teletext in Europe. The data rate of these signals is such that they cannot reliably be recorded on standard domestic video recorders (VCR) because of the restricted bandwidth, and even with "Hi-band" such as S-VHS recorders there can be reproduction problems caused by timing errors.
Much of the information contained within this data as transmitted is ephemeral and consequently, there would be little demand for it to be recorded along with the picture information. For example, if a transmitted program is recorded for later viewing this viewing may not take place for a period of several days or even possibly weeks. As a result transmitted information such as weather forecasts or travel information would be of no interest when the recording was replayed. There are, however, some classes of data transmitted which retain their usefulness over long periods or even indefinitely. One of these is the transmission of subtitles, for example for the hard of hearing. It is of course possible to decode these subtitles in the VCR at the time of recording and to record them superimposed in written form within the picture area (just as they would appear in normal viewing). This, however, has a number of disadvantages. First the viewer has to ensure that the subtitling feature is enabled at the time of recording the broadcast. Secondly, the subtitles are permanently "burnt into" the reproduced picture and hence cannot be dispensed with, which can be annoying for viewers who do not need them. Thirdly, a loss of sharpness of the subtitle text results from the limited bandwidth of the video recording/playback process. Fourthly, if recording of subtitles is requested when they are not in fact being transmitted, problems may occur in that some broadcasters may transmit a "subtitles not available" message which if "burnt-in" can obliterate picture information such as broadcast in vision subtitles.
Other data which may be usefully recorded, such as the time, date and program source, which could be obtained from the teletext service and a running total of frame count or time code could also be implemented.
The data need not, however, be in the teletext format or even transmitted during the vertical blanking interval. A possibility which has been considered is the use of available data capacity in the Nicam digital sound broadcasting services for an audio description signal. When decoded this signal provides a commentary on the picture for those having impaired sight. By recording the data from the decoder rather than the decoded audio signal, this could be selected or rejected on replay by the viewer. Clearly those without impaired sight would not wish to have this audio description signal permanently associated with the reproduced recording.